Seismic retrofit schemes for RCstructures and local–globalconsequencesG E Thermou1 and A S Elnashai2
1 Demokritus University of Thrace, Xanthi, Greece2 University of Illinois at Urbana-Champaign, IL, USA
SummaryA review of repair schemes for reinforcedconcrete frame buildings is presented in thispaper, within the context of global objectives ofthe intervention process. Local as well as globalintervention measures are discussed and theirtechnological application details outlined. Theeffect of the reviewed repair schemes onthe member, sub-assemblage and systemperformance are qualitatively assessed. Theimportant role of the foundation system in therehabilitation process is outlined and measuresthat are consistent with the super-structureintervention methods are given. The paperconcludes with a global assessment of the effect of
repair methods on stiffness, strength andductility, the three most important seismicresponse parameters, to assist researchers andpractitioners in decision-making to satisfy theirrespective intervention objectives. Theframework for the paper complies with therequirements of consequence-based Engineering,where the expected damage is addressed onlywhen consequences are higher than acceptableconsequences, and a cyclical process ofassessment and re-assessment is undertaken untilthe community objectives are deemed to besatisfied.
Key words: retrofit; repair/strengthening; rehabilitation; structural intervention; seismic upgrading
Prog. Struct. Engng Mater. 2006; 8:1–15
Published online 19 December 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pse.208
Introduction
In recent years, devastating earthquakes worldwideconfirmed the deficiencies of building structures. Theexperience gained from field observations and back-analysis led to improvement of the level of knowledgeand the evolution of seismic codes.
The interest of the research community is focusedon buildings that do not comply with current seismiccodes and exhibit deficiencies such as poor detailing,discontinuous load paths and lack of capacity designprovisions. Since such buildings comprise themajority of existing building stock, retrofitting is arather critical issue. Rehabilitation schemes that willprovide cost-effective and structurally effectivesolutions are necessary. Many intervention methodsused in the past have been revised and developed inthe light of the new seismic code requirements andnew methods often based on new materials (e.g. fiber-reinforced polymers FRPs) have been proposed.
In this paper, the term ‘rehabilitation’ is used as acomprehensive term to include all types of repair,retrofitting and strengthening that lead to reducedearthquake vulnerability. The term ‘repair’ is definedas reinstatement of the original characteristics of adamaged section or element and is confined todealing with the as-built system. The term‘strengthening’ is defined as intervention that lead toenhancement of one or more seismic responseparameters (stiffness, strength, ductility, etc.),depending on the desired performance.
Framework of seismic rehabilitation
Performance objectives are set depending on thestructural type, the importance of the building, its rolein post-earthquake emergencies, the economicconsequences of business interruption, its historical or
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cultural significance, the construction material andsocio-economic factors. They can be specified as limitson one or more response parameter such as stresses,strains, displacements, accelerations, etc. Clearly,different limit states have to be correlated to the levelof the seismic action, i.e. to the earthquake demandlevel.
The selection of the rehabilitation scheme and thelevel of intervention is a rather complex procedure,because many factors of different nature come intoplay. A decision has to be taken on the level ofintervention. Some common strategies are therestriction or change of use of the building, partialdemolition and/or mass reduction, addition of newlateral load resistance system, member replacement,transformation of non-structural into structuralcomponents and local or global modification(stiffness, strength and ductility) of elements andsystem. In addition, methods such as base isolation,provision of supplemental damping andincorporation of passive and active vibration controldevices may apply. The alternatives of ‘nointervention’ or ‘demolition’ are more likely theoutcomes of the evaluation if the seismic retrofit ofbuildings is quite expensive and disruptive.
Socio-economic issues have to be considered in thedecision of the level and type of intervention.Surprisingly, there are documented cases whereaesthetic and psychological issues dictate therehabilitation strategies. For example, in the MexicoCity earthquake of 19 September 1985, where externalbracing was popular, because it instilled a feeling ofconfidence in the occupants that significant andvisible changes have been made to the structure tomake it safer[1]. Cost vs importance of the structure isa significant factor, especially in the case that thebuilding is of cultural and/or historical interest. Theavailable workmanship and the level of qualitycontrol define the feasibility of the proposedintervention approach. The duration of work/disruption of use and the disruption to occupantsshould also be considered. The functionality andaesthetical compatibility of the intervention schemewith the existing building is an additionalengagement. Even the reversibility of the scheme incase it is not accepted on a long-term basis should betaken into account.
From a technical point of view the selection of thetype and level of intervention have to be based oncompatibility with the existing structural system andthe repair materials and technology available.Controlled damage to non-structural components andsufficient capacity of the foundation system areessential factors that are often overlooked. Issues suchas irregularities of stiffness, strength and ductilityhave to be considered in detail.
A convenient way to discuss the engineering issuesof evaluation and retrofit is to break down the processinto steps. The first step involves the collection of
information for the as-built structure. Theconfiguration of the structural system, reinforcementdetailing, material strengths, foundation system andthe level of damage are recorded. In addition, datarelevant to the non-structural elements (e.g. infillwalls) which play a significant role and influence theseismic response of structures are also compiled.Sources for the above information can become visits tothe site, construction drawings, engineering analysesand interviews with the original contractor. Therehabilitation objective is selected from various pairsof performance targets and earthquake hazard levels(i.e. supply and demand, or response and input pairs).The performance target is set according to anacceptable damage level (performance target).Building performance can be described qualitativelyin terms of the safety of occupants during and afterthe event, the cost and feasibility of restoring thebuilding to pre-earthquake condition, the length oftime the building is removed from service to effectrepairs, and the economic, architectural or historicimpacts on the larger community. Variations in actualperformance could be associated with unknowngeometry and member sizes in existing buildings,deterioration of materials, incomplete site data, andvariation of ground motion that can occur within asmall area and incomplete knowledge andsimplifications related to modeling and analysis. Inthe next phase, the rehabilitation method is selectedstarting with the selection of an analysis procedure.The development of a preliminary rehabilitationscheme follows (using one or more rehabilitationstrategies) the analysis of the building (includingrehabilitation measures), and the evaluation of theanalysis results. Further, the performance andverification of the rehabilitation design are conducted.The rehabilitation design is verified to meet therequirements through an analysis of the building,including rehabilitation measures. A separateanalytical evaluation is performed for eachcombination of building performance and seismichazard specified in the selected rehabilitationobjective. If the rehabilitation design fails to complywith the acceptance criteria for the selected objective,the interventions must be redesigned or an alternativestrategy considered.
Rehabilitation options
LOCAL INTERVENTION METHODS
The local modification of isolated components of thestructural and non-structural system aims to increasethe deformation capacity of deficient components sothat they will not reach their limit state as the buildingresponds at the required level. Local interventiontechniques are applied to a group of members thatsuffer from structural deficiencies and a combination
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of these techniques may be used in order to obtain thedesired behavior for a seismically designed structure.
Injection of cracks
Crack injection is a versatile and economical methodof repairing reinforced concrete (RC) structures. Theeffectiveness of the repair process depends on theability of the adhesive material (usually epoxies) topenetrate, under appropriate pressure, into the finecracks of the damaged concrete. Flexural cracks andshear cracks are mainly continuous and thereforeprovide unobstructed passages for the epoxy. On theother hand, longitudinal cracks, which develop alongreinforcing bars as a result of bond failure, are usuallydiscontinuous and narrow. Difficulties may occur inrepairing the steel-to-concrete bond by epoxyinjection.
This repair method can be used in minor(50.1 mm), medium (53 mm) size cracks, and largecrack widths (up to 5–6 mm). In case of larger cracks,up to 20 mm wide, cement grout, as opposed to epoxycompounds, is the appropriate material for injection(Fig. 1). In the first step of the application process,loose material is removed. For the more usual case ofepoxy injection, the surface trace of cracks is fullysealed with epoxy paste, leaving only surface-mounted plastic nozzles for injection. The spacing ofnozzles along the crack should be dictated by thedistance epoxy can travel prior to hardening (thisdistance depends on crack width and on the viscosityof the epoxy at the application temperature). Inmembers with dimensions larger than hardeningdistance, ports at both surfaces should be providedalong penetrating cracks. Injection is deemedcomplete for a portion of the crack when epoxy isexpelled from the next higher nozzle. Once the repairepoxy has set, the nozzles are bent and tied firmly.They can be cut flush and sealed with an epoxy-patching compound prior to rendering of the affectedmember.
Flexural tests on RC beams and beam–column jointsshow that the repair process not only eliminates the
unsightly appearance of wide cracks, but also restoresthe flexural strength and stiffness of the damagedmember[2,3]. Push-off tests (both static and dynamic)further indicate that concrete-to-concrete joints canregain their shear strength after being repaired byepoxy resin injection.
Shotcrete (Gunite)
Shotcrete is used as a repair method for RC andmasonry structures. There are two distinct types ofshotcrete; dry-mix and wet-mix. Shotcrete can beapplied to almost any surface; it can also be used incombination with other retrofit schemes (e.g. RCjacket). Because of its generally low water–cementratio and high-velocity impact, it achieves excellentbond to most competent surfaces. Deficiencies inshotcrete applicability usually fall into one of thefollowing five categories[4]: (i) failure to bond to thereceiving surface, (ii) de-lamination at constructionjoints or interfaces of various application layers, (iii)incomplete filling of the material behind thereinforcing steel, (iv) slough due to excess mixingwater (which can generate voids) and (v) weakinterface between the concrete and steel. The impactvelocity of the material to the application surface isdependent upon both the exit velocity and thedistance of the nozzle from the surface. Where bond isimportant, equipment must be at the proper impactangle of about 908 and reasonably close to theapplication surface. Further, the surface must be clean,sound and damp. When the shotcrete strikes theapplication surface (or other hard objects such asreinforcing steel), some of the larger and harderaggregate particles tend to ricochet. These particlesare referred to as rebound and are composedprimarily of the larger aggregate particles, althoughsome cement and water are included. Because of thenature of its composition, rebound is not capable ofobtaining significant strength and should not beallowed in the final work. Many factors affect theamount of rebound such as: (i) orientation of thereceiving surface, (ii) shotcrete mix design,(iii) amount of reinforcing steel embedment,
Fig. 1 Application of the: (a) epoxy resin; (b) cement grout injection in beam–column joints
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(iv) thickness of the cross-section, (v) impact velocity,and (vi) spraying technique.
Steel plate adhesion
Steel plate adhesion is mainly used in the case ofbeams. Both shear and flexural strength enhancementcan be achieved. When thick steel plates are needed, itis advisable to use several thin layers instead, tominimize interfacial shear stresses. A soundunderstanding of both the short- and long-termbehavior of the adhesive used is required. In addition,reliable information concerning the adhesion toconcrete and steel is required. The execution of thebonding work is also of great importance to achieve acomposite action between the adherents. Preventionof premature de-bonding or peeling of externallybonded plates is a most critical aspect of design[5–7].
Steel jacketing
The steel jacketing option involves the totalencasement of the column with thin steel platesplaced at a small distance from the column surface,with the ensuing gap filled with non-shrink grout[8,9].An alternative to a complete jacket (exemplified inFig. 2b,c) is the steel cage alternative[10,11]. Steel anglesare placed at the corners of the existing cross-sectionand either transversal straps or continuous steel platesare welded on them. In practice, the straps are oftenlaterally stressed either by special wrenches or bypreheating to temperatures of about 200–4008C, priorto welding. Any spaces between the steel cage and theexisting concrete are usually filled with non-shrinkgrout. When corrosion or fire protection is required, agrout concrete or shotcrete cover may be provided.
The corrugated steel jacketing technique can beapplied for the rehabilitation of columns and beam–column joints[12]. Deficient connections are encased bythe steel jacket and the gap between the concrete andthe steel jacket is filled with non-shrink grout. A gapis provided between the beam jacket and the columnface to minimize flexural strength enhancement of thebeam; which may cause excessive forces to develop inthe joint and column.
Externally bonded FRPs
The ease of application of FRP composites rendersthem attractive for use in structural applications;especially in cases where dead weight, space or timerestrictions exist. Although FRP composites can havestrength levels significantly higher than those of steeland can be formed of constituents such as carbon(CFRP), glass (GFRP), and aramid (AFRP) fibers, it isimportant to note that its use is often dictated bystrain limitations[13] (Fig. 3a). They are very sensitiveto transverse actions (i.e. corner or discontinuityeffects) and unable to transfer local shear (i.e.interfacial failure). Clearly, they carry no compressiveforces. Choosing the type of fibers, their orientation,their thickness and the number of plies, results in agreat flexibility in selecting the appropriate retrofitscheme that allows to target the strength hierarchy atboth local (i.e. upgrade of single elements) and global(i.e. achievement of a desired global mechanism)levels. In general, FRP composites behave in a linearelastic fashion to failure without any significantyielding or plastic deformation. Additionally, itshould be noted that unlike reinforcing steel, somefibers (such as carbon fibers) are anisotropic. Thisanisotropy is also reflected in the coefficient of
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thermal expansion in the longitudinal and transversedirections. The large differences in strength(transverse strength 5 longitudinal strength) andcoefficients of thermal expansion can result in bonddeterioration and splitting of concrete. Moreover,these can cause lateral stresses and low cycle fatigueunder repeated thermal cycling[15].
The effectiveness of strengthening depends on thebond conditions, the available anchorage length and/or the type of attachment at the FRP ends, thethickness of the laminates, among other lessimportant factors. According to experimental data,failure of the FRP reinforcement may occur either bypeeling off (de-bonding) through the concrete near theconcrete–FRP interface or by tensile fracture at a stresswhich may be lower than the tensile strength of thecomposite material, because of strengthconcentrations (e.g. at rounded corners or at de-bonded areas). In many cases, the actual failuremechanism is a combination of FRP de-bonding atcertain areas and fracture at others. The choice ofconstituents and details of the process used tofabricate the composite significantly affectenvironmental durability. Exposure to a variety ofenvironmental conditions can dramatically changefailure modes of the composites, even in cases whereperformance levels remain unchanged. In other cases,exposures can result in the weakening of the interfacebetween FRP composites and concrete, causing achange in failure mechanism and sometimes adramatic change in performance.
In the case of columns, shear failure, confinementfailure of the flexural plastic hinge region and lapsplice de-bonding can be accommodated by the use ofFRPs[16–18]. At this juncture it is important to stressthat none of these failure modes and associatedretrofits should be viewed separately, sinceretrofitting for one deficiency may only shift theproblem to another location and/or failure modewithout necessarily improving the overallperformance. For example, a shear-critical column,strengthened over the column center region withcarbon wraps, is expected to develop flexural plastic
hinges at column ends which, in turn, need to beretrofitted for the desired confinement levels.Furthermore, lap splice regions need not only to bechecked for the required clamping force to developthe capacity of the longitudinal columnreinforcement, but also for confinement and ductilityof flexural plastic hinge[17]. Shear and flexuralstrengthening of beams can be achieved by theapplication of either epoxy-bonded laminates orfabrics extending in the compression zone or epoxy-bonded FRP fabric wrapped around the beam[19–22]. Inthe case of beam–column joints, the jacket is designedto replace missing transverse reinforcement in thebeam–column joint[23–28]. The FRP technique can bealso used for strengthening walls[29].
Selective intervention methods
Where system-optimal performances dictateselectively modifying specific response parameters topre-defined levels, procedures for affecting singleparameters with no effect on others are called for. Theinitial development of ‘selective intervention’techniques, proposed by Elnashai[30] was first appliedto structural walls under static loading[31]. Furtherstudies applied the techniques to shaking table-testedwalls[32], and culminated with application to a full-scale four story RC building[33]. The fundamentalparameters governing structural responses totransverse actions in the inelastic range are: stiffness,strength and ductility. Consequently, selectiveintervention techniques are referred to as stiffness-,strength-, and ductility-only.
Stiffness-only intervention approaches may be usedin order to accommodate problems related to irregulardistribution of stiffness or to significant reduction ofstiffness due to cracking of concrete members. In thelatter case, if concrete crushing and buckling ofreinforcement bars do not occur the flexural strengthof the members will not necessarily be adverselyaffected.
Altering the sequence of plastic hinge formation toachieve a predetermined failure mode becomes anessential objective for seismic safety. This requires an
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Fig. 3 (a) Material properties[14]; application modes of (b) prefabricated shells; (c) FRP sheets
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increase in strength of strategically located members.Only a selective strength-only intervention can beeffective in addressing such deficiency.
Problems with lack of ductility supply may beconfronted by the application of ductility-onlyintervention methods. Lot of effort has been puttogether towards the investigation of alternativeductility-only retrofit schemes. Aboutaha et al.[34]
investigated the effectiveness of rectangular steeljackets for improving the ductility and strength ofcolumns with inadequate lap splice in thelongitudinal reinforcement. Several types of steeljackets were investigated, including rectangular solidsteel jackets with and without adhesive anchor bolts.A similar set of experiments was conducted by Avileset al.[35]. The models were deficient in the level ofconcrete confinement at foundation level and thusretrofitted with steel plate wrapping combined withanchor bolts. Saadatmanesh et al.[36] carried outexperimental work on the application of high-strengthFRP composite straps to retrofit bridge columns.Ghobarah et al.[37] investigated the effectiveness ofcorrugated steel jacketing for the seismic upgrading ofRC columns.
GLOBAL INTERVENTION TECHNIQUES
In case of systems with high flexibility or when nouninterrupted transverse load path is available thenglobal intervention techniques are considered. Themost well known global retrofit schemes arepresented hereafter.
RC jacketing
RC jacketing is one of the most commonly appliedmethods for the rehabilitation of concrete members.Jacketing is considered to be a global interventionmethod if the longitudinal reinforcement placed in thejacket passes through holes drilled in the slab andnew concrete is placed in the beam–column joint(Fig. 4). However, if the longitudinal reinforcement
stops at the floor level then RC jacketing is consideredas a member intervention technique. The mainadvantage of the RC jacketing technique is the factthat the lateral load capacity is uniformly distributedthroughout the structure of the building therebyavoiding concentrations of lateral load resistance,which occur when only a few shear walls areadded[38]. A disadvantage of the method is thepresence of beams which may require most of the newlongitudinal bars in the jacket to be bundled into thecorners of the jacket. Because of the presence of theexisting column, it is difficult to provide cross ties forthe new longitudinal bars, which are not at thecorners of the jacket.
To date, apart from qualitative guidelines providedin some Codes, no specific design rules exist fordimensioning and detailing of the jackets to reach apredefined performance target. The uncertainty withregard to bond between the jacket and the originalmember is another disadvantage. Of the many factorsinfluencing jacket performance, slip and shear-stresstransfer at the interface between the outside jacketlayer and the original member that serves as the coreof the upgraded element are overridingconsiderations[39].
The effectiveness of the method has been studied bymany researchers and supported by experimentalwork[38,40–42]. In cases where building are in closeproximity to one another, the method is modified andone-, two- or three-sided jacketing applies[43,44].
Addition of walls
Addition of new RC walls is one of the most commonmethods used for strengthening of existing structures.This method is efficient in controlling global lateraldrift, thus reducing damage in frame members.During the design process, attention must be paid tothe distribution of the walls in plan and elevation (toachieve a regular building configuration), transfer ofinertial forces to the walls through floor diaphragms,struts and collectors, integration and connection of the
Fig. 4 Reinforced concrete jacketing technique
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wall into the existing frame buildings and transfer ofloads to the foundations. Added walls are typicallydesigned and detailed as in new structures. To thisend, in the plastic hinge zone at the base they areprovided with boundary elements, well-confined anddetailed for flexural ductility. They are also capacity-designed in shear throughout their height and over-designed in flexure above the plastic hinge region(with respect to the flexural strength in the plastichinge zone, not the shear strength anywhere), toensure that inelasticity or pre-emptive failure will nottake place elsewhere in the wall before plastic hingingat the base and that the new wall will remain elasticabove the plastic hinge zone.
The most convenient way to introduce new shearwalls is by partial or full infilling of strategicallyselected bays of the existing frame[45]. If the wall takesup the full width of a bay, then it incorporates thebeams and the two columns, the latter acting as itsboundary elements (Fig. 5). In case only the web of thenew wall needs to be added, sometimes byshotcreting against a light formwork or a partitionwall is performed. In the latter case, shotcrete isnormally used for increased adhesion betweenthe existing and the added material. An alternativeto the cast-in-place infill wall technique is the additionof pre-cast panels. The pre-case infill wall systemshould be designed to behave monolithically, and theinfill wall should be designed with sufficient shearstrength to develop flexural yielding at the base of thewall[46].
A major drawback of the addition of walls is theneed for strengthening the foundations to resist theincreased overturning moment and the need forintegrating the wall with the rest of the structure.Foundation intervention is usually costly and quitedisruptive, thus rendering the application of thistechnique unsuitable for buildings without an existingadequate foundation system.
External buttresses
To reduce or eliminate the disruption to the use of abuilding, external buttresses may be constructed toincrease the lateral resistance of the structure as awhole. Such an intervention scheme, in common withthe construction of RC walls, requires a newfoundation system. The foundation scheme wouldpossibly be eccentric footings (eccentric with respectto the axis of the buttress to avoid excavation underthe building). The two most intricate problems instrengthening by building a set of external buttressesare: (i) the buttress stability may be critical since it isnot actually loaded vertically downwards in the sameway that the structure is. The vertical action on thebuttress is only its own weight. This increases thepossibility of uplifting of the foundations and mayeven cause over-turning, (ii) the connections betweenthe buttresses on the one hand and the building on theother is far from straightforward. To insure fullinteraction and load sharing when the structure issubjected to lateral actions, the buttress should beconnected to the floors and columns at all levels. Theconnection area will be subjected to unusual levels ofstresses that require special attention.
Steel bracing
Steel bracing can be a very effective method for globalstrengthening of buildings. Some of the advantagesare the ability to accommodate openings, the minimaladded weight to the structure and in the case ofexternal steel systems minimum disruption to thefunction of the building and its occupants.
Alternative configurations of bracing systems maybe used in selected bays of a RC frame to provide asignificant increase in horizontal capacity of thestructure. Concentric steel bracing systems have beeninvestigated for the rehabilitation of non-ductilebuildings by many researchers[47–50]. Using theeccentric steel bracing in the rehabilitation of RC
Fig. 5 Cast-in-place infill walls
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structures has lagged behind concentric steel bracingapplications due to the lack of sufficient research andinformation about the design, modeling and behaviorof the combined concrete and steel system. Furtherresearch is needed in several areas such as testing ofthe RC beam–steel link connection details and designas well as the development and implementation oflink elements models in analysis software[51]. Post-tensioned steel bracing can be used for the seismicupgrading of infilled non-ductile buildings limitedto low-rise and squat medium-rise buildings[52].The method was successfully used byMiranda & Bertero[53] to effectively upgradethe response of low-rise school buildings inMexico.
Base isolation
Seismic isolation is mostly adopted for rehabilitationof critical or essential facilities, buildings withexpensive and valuable contents and structureswhere performance well above performance levels isrequired. Seismic isolation system significantlyreduces the seismic impact on the buildingstructure and assemblies. Generally, the isolationdevices are inserted at the bottom or at the top ofthe first floor columns. Retrofitting mostly requirestraditional intervention; in the first case theaddition of a floor in order to connect all the columnsabove the isolators while in the second case thestrengthening of the first floor columns (enlarging ofthe cross-sections, addition of reinforcing bars orconstruction of new resistant elements). Nevertheless,inserting an isolator within an existing column is notso simple because of the necessity of cutting theelement, temporarily supporting the weight of theabove structure, putting in place the isolators andthen giving back the load to the column, withoutcausing damages to persons and to structural andnon-structural elements.
Recently, efforts have been made to extend thisvaluable earthquake resistant strategy to inexpensivehousing and public buildings[54]. The results of a jointresearch program conducted by the InternationalRubber Research and the Development Board(IRRDB) of United Kingdom show that the methodcan be both cost effective and functional for theprotection of small buildings in high seismicityregions. A comparative study conducted by Bruno &Valente[55] on conventional and innovative seismicprotection strategies concluded that base isolationprovides higher degrees of safety than energydissipation does, regardless of the type of devicesemployed. Moreover the comparison betweenconventional and innovative devices showed thatshape memory alloys-based devices are far moreeffective than rubber isolators in reducing seismicvibrations.
Effect of retrofit on global response
Development of a complete strategy guiding theretrofit solution through established objectives orcriteria is an ongoing effort of the earthquakeengineering research community. In general, seismicrehabilitation may aim to either recover or upgradethe original performance or reduce the seismicresponse[56]. In the first case, the retrofit schemes thatwill be chosen have to reinstate the structuralcharacteristics at member level and have negligibleimpact on the global response. The crack injection(epoxy resin injection or grout injection) techniqueand the member replacement (substitute part of thedamaged member) may apply.
When the seismic demand is to be reduced, this canbe achieved by adopting base isolation techniques orby providing the structure with supplementaldissipation devices. Reducing the masses at each storylevel accommodating irregularities in the massdistribution along the height of the building is aneffective way of reducing seismic demand. In manycases (in areas of rapid economic and industrialdevelopment) the functionality of residentialbuildings is changed and they are used for eitherstorage or installation of heavy industrial equipment.Due to the discontinuity in mass distribution theparticular floors are susceptible to failure. Moreover,the total or partial demolition of the top stories ofstructures can result in the reduction of the period soas to comply with the seismic demand.
In the case of the seismic upgrading, the aim of theretrofit strategy as an operational framework is tobalance supply and demand. The supply refers to thecapacity of the structural system, which has to beassessed in detail before selecting the interventionscheme. The demand is expressed by either a codedesign spectrum or a site-specific set of records as afunction of period and shape of vibrationcharacteristics of the upgraded system. By modifyingstrength, stiffness or ductility of the system alternativeretrofit options are obtained, as shown in Fig. 6.Ductility enhancement applies to systems with poordetailing (sparse shear reinforcement, insufficient lapsplicing), stiffness and strength enhancement tosystems with inherently low deformation capacity (soas to reduce displacement demand), whereas stiffness,strength and ductility enhancement apply to systemswith low capacity or where seismic demand is high[57].
An effective retrofit scheme for dealing withductility deficiencies of the structural system is FRPjacketing. Assuming that the as-built system has beendesigned according to the strong-column weak-beammechanism approach, FRP jacketing of the verticalelements provides additional confinement of theexisting columns. The effectiveness of the methoddepends on reassuring that slip of longitudinal bars ofthe existing column will not occur and to the bondconditions between the existing member and the new
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material. The behavior of the retrofitted structure isrepresented herein, for demonstration purposes, bythe behavior of the retrofitted 2-story, 2-bay RC frameshown in Fig. 7a. The span length of the frame is 5 m,while the story height is 2.7 m. The columns havedimensions 0.40� 0.40 m, longitudinal reinforcementratio rl ¼ 0:77% and confinement reinforcementvolumetric ratio rsw ¼ 0:22% (#6/0.15 m). Thematerial strengths of the existing structure are C16and S300. Using FRP jacketing in order to increase theconfinement factor to a value of, K ¼ 2, and byperforming pushover analysis by ZEUS-NL[58] the topdisplacement at ultimate is increased by 122%. If theseismic upgrading targets the modification ofstiffness, strength and ductility levels, RC jacketingcan be chosen as a retrofit solution. The response ofthe retrofitted structure depends on the characteristicsof the jacket such as longitudinal reinforcement,confinement reinforcement and material strengths. Inthis case the effectiveness of the solution schemedepends on the continuity between the existing andthe new material and the effectiveness of anchorage ofthe additional reinforcement of the jacket. Theresponse of the retrofitted frame is shown in Fig. 7bfor two alternative jacket configurations J1 and J2,respectively. In both cases the jacket dimensions are0.50� 0.50 m, the material strength characteristics C20and S400, but in the first case ( J1) the total longitudinalreinforcement ratio of the jacketed cross-section is rlj
¼ 0:85% and confinement reinforcement volumetricratio rswj ¼ 0:93% (#10/0.075 m), while in the second(J2) the total longitudinal reinforcement ratio of thejacketed cross-section is rlj ¼ 1:31% and confinementreinforcement volumetric ratio rswj ¼ 1:40%
(#10/0.050 m). The first jacket configuration ( J1)increases the strength level (maximum base shear) by55%, while the second ( J2) by 89% (Fig. 7b). In bothcases the ductility level is increased dramatically.
The response modification of the existing structuralsystem may be achieved by adopting a combination ofthe pre-described local and global interventiontechniques. The strategic use of the retrofit schemescan accommodate all deficiencies observed at localand/or global level and result in a cost- and time-effective solution.
SYSTEM-LEVEL DEFICIENCIES
System-level deficiencies such as eccentricities ofstiffness (or strength) and mass in both plan andelevation are common in existing structures. This classof deficiency is a consequence of old constructionpractices (poor level of confinement details, negligiblematerial-property control). Due to lack of specificguidelines most retrofit strategies adopted in practiceare based mainly on experience and in few cases onsimple analysis (with the exception of majorstructures in high seismicity regions, such asCalifornia and parts of Japan). Recent earthquakeshave demonstrated that the rehabilitation measurestaken in the past failed to meet the retrofitperformance objectives. In many cases, misuse of theretrofit solution schemes was observed. A major issueseems to be the difficulty in understanding theinteraction between the retrofit scheme and theexisting structural system. A sound understanding ofthe response of the existing structural system and aclear definition of the performance objectives of the
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(kN
)
Original-Frame
Retrofitted-FRPs
(a) (b)
Fig. 7 (a) Ductility enhancement}FRP jackets; (b) stiffness, strength and ductility enhancement}RC jackets
Ductility enhancement
∆u
Roof Displacement Roof Displacement Roof Displacement
BaseShear
existingstructure
rehabilitated structure
(a) (c)
BaseShear
existing structure
Stiffness, strength & ductility enhancement
rehabilitatedstructure
∆u
∆V∆V
BaseShear
existingstructure
Stiffness & strength enhancement
rehabilitatedstructure
(b)
Fig. 6 Alternative retrofit strategies for seismic upgrading[57]
Copyright & 2005 John Wiley & Sons, Ltd. Prog. Struct. Engng Mater. 2006; 8:1–15
retrofit strategy are necessary before embarking onthe design of the retrofit solution.
Vertical irregularities (irregularities along thevertical axis) are due to either irregular distribution ofmass or stiffness along the height of the building. Asmentioned above, buildings may be used for adifferent purpose from their original intendedfunction. The concentration of mass at a particularstory attracts higher seismic forces and results in thecreation of a soft story.
Vertical irregularities may also be due to irregularstiffness distribution. A special case is the soft-storymechanism. A common structural configuration(typical of the construction practice in SouthernEurope) susceptible to a soft-story failure mechanismis the pilotis frame. The ground story used forcommercial facilities is an open frame (bare frame),while the stories above are infilled. Under lateralloading, the ground-story columns have to resist thelarge base shear which leads to large story driftconcentrated in the first story. The large demandincreases progressively due to second-order effects,often leading to the collapse of the structure in a soft-story mechanism.
Observation of practical application has shown thatthere is lack of clarity with regard to the way soft-story mechanism is treated. Increasing the stiffness ofthe ground level only to reach the stiffness of theinfilled floor above is not the correct approach, sincethe stiffness of the floor above depends on thestrength of the masonry infills. In a future earthquake,as soon as the masonry infills start cracking, or evenshed out-of-plane, the localization of damage istransferred to the story above. The retrofit strategyshould aim to develop a uniform distribution ofstiffness along the height of the building. RC jacketingand the addition of RC walls can be effective retrofitsolutions provided they are applied to achieve a targetdisplaced shape.
Horizontal irregularities (irregularities in the planof the structure) are due to the eccentricity betweenthe centers of mass and stiffness. The unevendistribution of stiffness may be the result ofarchitectural (e.g. L-shaped buildings) or functional(e.g. facade of commercial buildings) features. Theposition of the elevator shaft walls plays an importantrole in the distribution of stiffness in plan. Walls andcolumns have to be placed in strategic positions inorder to accommodate irregularities. The retrofitstrategy should aim to balance the stiffness ormass irregularities in plan. The addition of newelements (e.g. RC walls, external buttresses)may be used to advantage in addressing planirregularities.
The effect of the various intervention schemes atlocal and global level and some useful comments withregard to the effectiveness of the method andparameters that should be taken into account in thedesign phase are presented in the appendix (Table A1).
ROLE OF FOUNDATION SYSTEM
Seismic upgrading of the super-structure has a directeffect on the demand imposed on the existingfoundation system. Structural requirements maydictate considerable strength enhancement inlocations that are connected directly to thefoundations. Capacity design principles immediatelydictate that foundation strengthening is needed.Moreover, parameters such as soil conditionsand soil–structure interaction play an importantrole in foundation-strengtheningprojects.
Old buildings mainly supported by isolatedfootings and in fewer cases by combined footings areweak or flexible compared to the current seismicdesign philosophy. In the majority of cases, thefoundation system along with the rest of the structureare representative of construction practices adopted inthe past and may be susceptible to a number ofdifferent modes of brittle failure.
Retrofit strategies may aim at either strengtheningthe existing foundation system and/or addingsupplemental foundation elements (footings or piles).Larger spread footings can distribute the load andadditional reinforcement can increase their shear andbending resistance. The incorporation of existingfootings into grade beams or mats, which can spreadload over a larger soil area and activate the gravityloads in other columns in the resistance of theoverturning moments and uplift forces, is anotheroption. Projects involving the addition of grade beamsor increased size of spread footings usually requireexcavation under difficult circumstances and there aredifficulties in pinning or attaching the existingfootings to the new elements[59]. Moreover, piles maybe added to improve the overturning resistance.Adding piles along the perimeter of the building canbe an easier task from an economical andconstructional point of view compared to the casewhere piles are added under the interior ofthe buildings.
The selection of the RC jacketing as the retrofitsolution for the super-structure results in a uniformdistribution of stiffness. The retrofit of the foundationsystem can be relatively easily accommodated byextending the jacketing to the foundation level(Fig. 8). On the other hand, the addition of newelements (e.g. RC walls, external bracings) may addstrength and stiffness to the building at criticallocations. In these cases, greater demands on thefoundation system are placed. The shear transmittedbetween the soil and the strengthened structure maybe higher because of the increased strength andstiffness of the structure[59]. Stiff structuralcomponents generate large bending moments at thebase. Large overturning movements may cause largedynamic axial forces to develop in the columnsof braced frames or at the boundary elements ofshear walls.
EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS10
Copyright & 2005 John Wiley & Sons, Ltd. Prog. Struct. Engng Mater. 2006; 8:1–15
A foundation system that allows the developmentof hinges in the super-structure is vital for the stabilityof structural and non-structural components. Seismicupgrading of foundations is usually a disruptiveprocess. The cost varies depending on the type andthe level of intervention. In cases where piles have tobe installed in the existing system the cost maydominate the total seismic retrofit project.
Conclusions
Numerous retrofit schemes adopted in practice for theseismic upgrading of old and substandard reinforcedconcrete buildings are presented. A multiplicity offactors influence the selection of the retrofit solutionand therefore no general rules apply. To aid in theselection, the effectiveness of the retrofit schemes and
their interaction at local and global level is explored.The main system-level deficiencies (vertical andhorizontal irregularities) are presented and relatedmodeling issues are clarified. The impact ofstrengthening of the super-structure on thefoundation system and the alternative retrofit optionsfor the foundation system are discussed. The paperconcludes with a table summary of the retrofitoptions, motivation for use, local and global effects,technological and design requirements, intended toprovide a quicklook guide to potential users.
Appendix A
The summary of the effect of retrofit on local andglobal response is shown in Table A1.
Fig. 8 Strengthening of footings}RC jacketing
SEISMIC RETROFIT SCHEMES 11
Copyright & 2005 John Wiley & Sons, Ltd. Prog. Struct. Engng Mater. 2006; 8:1–15
.............................................................................................................................................................................................................................
Tab
leA
1Su
mm
ary
of
the
effe
ctof
retr
ofit
on
loca
lan
dgl
obal
resp
onse
Meth
od
Defi
cie
ncy
typ
eL
ocal
eff
ect
Glo
bal
eff
ect
Tech
no
logy
co
nsi
dera
tio
ns
Desi
gn
co
nsi
dera
tio
ns
Inje
ctio
nof
crac
ksSh
ear
or
shea
r-fle
xura
lcr
acks
Flex
ura
lst
rengt
han
dst
iffnes
sre
stora
tion.Sh
ear
stre
ngt
his
rega
ined
inco
ncr
ete-
to-
concr
ete
join
ts
Rep
air
met
hod
}no
modifi
cation
of
the
resp
onse
of
the
ori
ginal
stru
cture
The
qual
ity
and
the
envi
ron-
men
taldura
bili
tyof
the
mat
eria
lsuse
dpla
yan
import
ant
role
.T
he
adhes
ive
mat
eria
lsh
ould
pen
etra
tein
toth
efin
ecr
acks
of
the
dam
aged
concr
ete
and
infil
lal
lth
evo
ids
Red
uct
ion
fact
ors
for
concr
ete
stre
ngt
hm
aybe
use
dto
take
into
acco
unt
any
unce
rtai
nty
rega
rdin
gth
eef
fect
iven
ess
of
the
met
hod
and
qual
ity
of
mat
e-ri
als
Shotc
rete
(Gunite)
Exte
nsi
vecr
ack
pat
tern
sat
concr
ete
mem
ber
sor
mas
onry
;co
nve
rtin
gnon-s
truct
ura
lto
stru
ctura
lw
alls
Rei
nst
atem
ent
of
the
ori
ginal
char
acte
rist
ics
of
the
elem
ent
for
repai
r;in
crea
sein
forc
edem
and
ifap
plie
das
are
trofit
ting
option
Min
imum
effe
ctw
hen
applie
das
are
pai
rm
ethod
ifla
yer
isve
ryth
ink
and
with
wir
em
esh
only
.C
om
ple
tech
ange
of
resp
onse
when
applie
doth
erw
ise
Judic
ious
atte
ntion
tosu
rfac
ecl
eanlin
ess.
Mix
des
ign
iscr
itic
al.Exper
ience
dper
sonnel
are
nec
essa
ry
The
applie
dla
yer
of
concr
ete
pro
vides
adeq
uat
est
rengt
h.
Itis
use
doft
enin
com
bin
atio
nw
ith
oth
erre
trofit
schem
es(e
.g.
RC
jack
etin
g).
Am
ount
of
rein
forc
e-m
ent
and
thic
knes
sof
laye
rdic
tate
slo
calan
dgl
obal
effe
cts
Stee
lja
cket
ing}
pla
tead
hes
ion
Insu
ffic
ient
shea
rst
rengt
han
dduct
ility
due
toold
type
of
det
ailin
g(s
par
seco
nfin
emen
tre
info
rcem
ent,
insu
ffic
ient
lap
splic
ing)
Jack
etin
g:D
eform
atio
nca
pac
ity
isin
crea
sed
Pla
tead
hes
ion:Sh
ear
and
flexura
lst
rengt
hen
han
cem
ent
Def
orm
atio
nca
pac
ity
isen
han
ced.St
rengt
hca
pac
ity
may
be
incr
ease
dor
rem
ain
the
sam
edep
endin
gon
the
effe
ctof
the
retr
ofit
schem
eat
loca
lle
vel
The
effe
ctiv
enes
sof
the
met
hod
isre
late
dto
the
type
of
grouts
use
dfo
rin
fillin
gth
ega
pbet
wee
nth
est
eelja
cket
and
the
exis
ting
mem
ber
.T
he
bondin
gw
ork
isof
grea
tim
port
ance
toac
hie
vea
com
posi
teac
tion
bet
wee
nth
ead
her
ents
Bef
ore
dec
idin
gfo
rst
eel
jack
et-
ing
pre
mat
ure
failu
redue
tooth
erm
echan
ism
s(e
.g.
pull-
out
ofth
elo
ngi
tudin
alre
info
rcem
ent
of
the
exis
ting
mem
ber
)sh
ould
be
pre
vaile
d.
Stee
lja
cket
should
be
consi
der
edas
additio
nal
con-
finem
ent
rein
forc
emen
t,w
hile
stee
lpla
tes
adher
edat
the
bot-
tom
flange
ofbea
ms
asad
ditio
nal
bott
om
rein
forc
emen
tFR
Pja
cket
ing
Insu
ffic
ient
shea
rst
rengt
han
dduct
ility
due
toold
type
of
det
ailin
g(s
par
seco
nfin
emen
tre
info
rcem
ent,
insu
ffic
ient
lap
splic
ing)
Colu
mns:
Def
orm
atio
nca
pac
ity
isen
han
ced
Bea
ms:
Shea
ran
dfle
xura
lst
rengt
hen
ing
Bea
m–co
lum
njo
ints
:Sh
ear
failu
reis
elim
inat
edin
connec
tions
Duct
ility
and
shea
rst
rengt
hat
stru
ctura
lle
velar
eim
pro
ved
Exposu
reto
ava
riet
yof
envi
ronm
enta
lco
nditio
ns
can
dra
mat
ical
lych
ange
failu
rem
odes
of
the
com
posi
tes,
even
inca
ses
wher
eper
form
ance
leve
lsre
mai
nunch
ange
d.H
igh
qual
ity
contr
olis
requir
ed.T
he
bondin
gw
ork
isve
ryim
port
ant
The
effe
ctiv
enes
sdep
ends
on
the
anch
ora
geco
nditio
ns
of
the
longi
tudin
alre
info
rcem
ent
ofth
eex
isting
mem
ber
.Lim
itat
ions
due
tost
ress
conce
ntr
atio
ns
should
be
consi
der
edin
the
des
ign
phas
e.FR
Pla
yers
are
equiv
alen
tto
additio
nal
confin
emen
t
Sele
ctiv
ein
terv
ention
met
hods
The
dam
age
pat
tern
vari
esdep
endin
gon
the
def
icie
nt
par
amet
er
Incr
ease
of
stiff
nes
s,st
rengt
hor
duct
ility
Stru
ctura
lre
sponse
can
be
tuned
tom
eet
the
per
form
ance
obje
ctiv
es
Exper
ience
dper
sonnel
are
requir
edin
the
exec
ution
phas
eR
efin
edm
odel
ing
isre
quir
edin
ord
erto
take
into
acco
unt
the
incr
ease
of
the
spec
ific
par
a-m
eter
.Spec
ializ
eddes
ign
expre
s-si
ons
nec
essa
ryR
Cja
cket
ing
Insu
ffic
ient
late
ralst
rengt
h,
insu
ffic
ient
def
orm
atio
nIf
the
jack
etis
applie
dat
floor
leve
l,both
axia
lan
dsh
ear
Ifth
eja
cket
continues
bet
wee
nsu
cces
sive
floors
,T
he
unce
rtai
nty
with
rega
rdto
bond
bet
wee
nth
eja
cket
The
resp
onse
ism
odifi
edto
stro
ng-
colu
mn
wea
k-bea
m
EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS12
Copyright & 2005 John Wiley & Sons, Ltd. Prog. Struct. Engng Mater. 2006; 8:1–15
.............................................................................................................................................................................................................................
capac
ity
and
stiff
nes
sdis
continuity
bet
wee
nsu
cces
sive
floors
stre
ngt
hof
the
colu
mn
are
impro
ved,w
hile
flexura
lst
rengt
han
dst
rengt
hof
the
bea
m–co
lum
njo
ints
rem
ain
the
sam
e
stiff
nes
s,st
rengt
han
dduct
ility
are
enhan
ced
and
the
ori
ginal
mem
ber
isac
com
modat
edby
the
use
of
monolit
hic
fact
ors
for
the
estim
atio
nof
the
def
orm
atio
nan
dst
rengt
hca
pac
ity
of
the
com
posi
tem
ember
mec
han
ism
with
dis
tinct
pla
stic
hin
gere
gions.
The
seis
mic
de-
man
dis
incr
ease
ddue
tosh
iftof
the
per
iod.
Unifo
rmdis
trib
ution
of
stre
ngt
han
ddef
orm
atio
nca
pac
ity
isat
tain
ed.
Exte
nsi
on
of
jack
etin
gto
foundat
ion
leve
lm
aybe
nec
essa
ryA
dditio
nof
wal
lsor
exte
rnal
butt
ress
es
Insu
ffic
ient
late
ralst
iffnes
san
dst
rengt
h,to
rsio
nal
lyunbal
ance
dst
ruct
ure
s
Def
orm
atio
ndem
and
atm
ember
leve
lis
dec
reas
ed,
while
stre
ngt
hdem
and
may
be
incr
ease
d.H
igh
dem
and
atco
nnec
tion
bet
wee
nex
isting
stru
cture
and
wal
lsor
butt
ress
esis
gener
ated
Glo
bal
late
raldri
fts
are
contr
olle
d.C
onsi
der
able
stre
ngt
han
dst
iffnes
sar
ead
ded
toth
eex
isting
stru
ctura
lsy
stem
.R
esultin
gsy
stem
isto
tally
diff
eren
tfr
om
the
ori
ginal
stru
cture
requir
ing
full
reas
sess
men
t
Inth
eca
seof
infil
lw
alls
,th
ein
terf
ace
bet
wee
nth
eex
isting
and
the
new
elem
ent
should
be
chec
ked.A
new
foundat
ion
syst
emis
nec
essa
ryif
wal
ls(u
sual
lyG
-shap
edin
the
per
imet
erof
the
build
ing)
or
butt
ress
esar
ead
ded
.In
any
case
,th
eex
isting
foundat
ion
syst
emnee
ds
tobe
stre
ngt
hen
edto
resi
stth
ein
crea
sed
ove
rturn
ing
mom
ent
and
the
larg
erw
eigh
tof
the
stru
cture
Acr
itic
alas
pec
tin
the
des
ign
phas
eis
toin
sure
full
inte
ract
ion
and
load
shar
ing
bet
wee
nth
eex
isting
stru
ctura
lsy
stem
and
the
new
one
(infil
l,ex
tern
alw
alls
or
butt
ress
es).
Connec
tors
should
be
pla
ced
atflo
or
leve
lan
dbeh
ave
elas
tica
llyfo
rth
edes
ign
eart
hquak
e.St
rengt
hen
ing
of
exis
ting
hori
zonta
lm
ember
sm
aybe
requir
ed.R
esponse
mod-
ifica
tion
of
the
syst
emfr
om
shea
rto
cantile
ver
type
isat
-ta
ined
with
ash
iftin
per
iod.
Stra
tegi
cdis
trib
ution
of
wal
lsm
ayac
com
modat
ean
ysy
stem
-le
veldef
icie
nci
esSt
eelbra
cing
Insu
ffic
ient
late
ralst
iffnes
san
dst
rengt
hH
igh
leve
lsof
forc
em
aybe
intr
oduce
dat
bra
ceen
ds
and
connec
tions
bet
wee
nbra
cem
ember
san
dex
isting
stru
cture
Late
ralst
iffnes
san
dst
rengt
hof
the
exis
ting
stru
cture
are
incr
ease
d.A
dditio
nal
ener
gydis
sipat
ion
ispro
vided
Inst
alla
tion
of
post
-ten
sioned
bar
sca
nsi
gnifi
cantly
modify
the
dis
trib
ution
of
inte
rnal
forc
esof
exis
ting
RC
mem
ber
s.B
raci
ng
mem
ber
ssh
ould
be
des
igned
tobeh
ave
ina
duct
ilem
anner
The
late
ralst
rengt
hofth
eex
ist-
ing
mem
ber
sm
aybe
adve
rsel
yaf
fect
edby
the
leve
lof
axia
lfo
rces
induce
dby
the
stee
lbra
ces.
Stre
ngt
hen
ing
of
col-
um
ns,
bea
ms
and
bea
m–co
lum
njo
ints
of
bra
ced
bay
snee
ded
for
the
adeq
uat
eper
form
ance
ofth
ebra
cing
syst
em.
Foundat
ion
sys-
tem
should
withst
and
the
in-
crea
sed
stre
ngt
han
dst
iffnes
sef
fect
sB
ase
isola
tion
Reh
abili
tation
of
critic
alor
esse
ntial
faci
litie
sT
he
seis
mic
impac
ton
stru
ctura
lan
dnon-s
truct
ura
lco
mponen
tsis
reduce
d
The
seis
mic
ener
gyis
abso
rbed
by
isola
tion
dev
ices
inse
rted
atth
ebott
om
or
atth
eto
pof
the
first
floor
colu
mns
Spec
ializ
eden
ginee
ring
exper
tise
isnec
essa
ryT
her
eis
no
nee
dfo
rre
trofit
ting
the
upper
par
tof
the
stru
cture
.T
he
equip
men
tsh
ould
be
pro
-vi
ded
with
capab
ilities
tow
ith-
stan
dth
eex
pec
ted
larg
ehori
zonta
ldis
pla
cem
ent
bet
wee
nth
efo
undat
ion
and
the
super
-st
ruct
ure
SEISMIC RETROFIT SCHEMES 13
Copyright & 2005 John Wiley & Sons, Ltd. Prog. Struct. Engng Mater. 2006; 8:1–15
Acknowledgements
The first author was funded by the EuropeanResearch Project SPEAR (Seismic PerformanceAssessment and Rehabilitation, Contract NumberG6RD-CT-2001-00525) through Imperial College ofScience, Technology and Medicine, London. Thecontribution of the second author was partiallyfunded by the US National Science Foundationthrough the Mid-America Earthquake Center, AwardNumber EEC 97-01785.
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G E Thermou
Civil Engineer, Reinforced Concrete Laboratory,
Department of Civil Engineering,
Demokritus University of Thrace,
Xanthi 67100, Greece
E-mail: [email protected]
A S Elnashai
Willett Professor of Engineering,
Director, Mid-America Earthquake Center,
Department of Civil and Environmental Engineering,
University of Illinois at Urbana-Champaign,
2129e Newmark Civil Engineering Laboratory, MC-250,
205 North Mathews Avenue,
Urbana, IL 61801, USA
E-mail: [email protected]
SEISMIC RETROFIT SCHEMES 15
Copyright & 2005 John Wiley & Sons, Ltd. Prog. Struct. Engng Mater. 2006; 8:1–15